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Altered Mafic Tuff Mounds From the Upper Cretaceous of Central Texas: The First Economical Unconventional Reservoir?


The Upper Cretaceous Balcones Igneous Province of central Texas is a suite of silica-undersaturated, alkalic igneous rocks including both intrusive and extrusive rocks. This study documents the post-eruptive alteration and porosity changes present in the extrusive rocks. The extrusives occur mainly as thick mounds of lapilli (>2 mm particles) tuff generated by submarine volcanoes. Mound thicknesses up to 850 ft have been observed. Mounds occur within sedimentary rocks, primarily carbonates, and have been a drilling target since 1915. Previous researchers have noted a broad range of intrusive compositions and it is suspected that the associated tuffs will show a similar range of compositions. Some previous researchers have mistakenly characterized the tuffs as being “serpentine.” This study focuses on two cores from different tuff mounds in the northern part of the province (Williamson and Caldwell Co.). Core depths range from 1501 to 2356 ft. Preliminary work on samples from the cores examined porosity, permeability, texture, and mineralogy. Many volcanic textures are preserved in the tuffs, despite widespread alteration. Minerals and volcanic glass are altered primarily to smectite and/or chlorite and in some cases titanite (sphene) and phosphate. Phenocrysts, presumably of olivine, are completely replaced. Alteration style and intensity varies between adjacent lapilli suggesting heterogeneous origin for material. Sporadic layering within the cores is primarily defined by differences in cementation and less by changes in particle size. Steep inclined layering in some intervals suggests slumping of material. Porosities up to 30% are present, highlighting the excellent hydrocarbon storage potential of these rocks. Pores are present in four locations; between lapilli, in partially cemented vesicles, in partially replaced phenocrysts, and between phyllosilicates replacing volcanic glass. Pore sizes extend from nanometers to millimeters in diameter. Most pore throats based on MICP are less than 250 nm in diameter and from a micropore system. Pore space, both between ash grains and lapilli and in vesicles, is lined with combinations of smectite, chlorite, pectolite, xonotlite and rarely calcite. One core is more Na-rich and has more smectite and pectolite, the other core has less Na, and has more chlorite and xonotlite. Klinkenberg permeability values for a limited sample set are in the range of one to a tenth of a millidarcy.